1. Field of the Invention
The invention relates to a yarn draw-off nozzle for an open-end spinning station of a rotor spinning machine, with spirally extending yarn deflection points or locations being disposed in a yarn feed region for supporting yarn to be drawn off.
In open-end rotor spinning, in which a spinning rotor is used as a twister, a problem which occurs is that the actual yarn twist does not uniformly run into the existing end of the yarn. In open-end rotor spinning, the actual yarn twist is imparted substantially to the piece of the yarn between the yarn draw-off nozzle and the yarn draw-off device. The yarn twist only incompletely runs into or enters the piece of yarn ahead of the draw-off nozzle, that is the piece of yarn that extends from the yarn draw-off nozzle to the rotor groove. That piece of yarn often has less twist than the normal yarn, so that the spinning stability suffers as a result.
In the industry, yarn draw-off nozzles that are provided with a surface structure such that the friction of the yarn on the yarn draw-off nozzle produces a false twist are therefore often used. In that way, the twist in the segment of yarn between the yarn draw-off nozzle and the rotor groove is increased.
Swiss Patent No. 503 127, for instance, discloses a yarn draw-off nozzle with a spiral structure in the yarn feed region. That spiral is constructed in the form of either a groove or a rib.
With yarn draw-off nozzles constructed in that way, the yarn rests on riblike protrusions that extend spirally from a bore in the draw-off nozzle to the rim of its mouth. The direction of the spiral extends in such a way that depending on the rotational direction of the yarn, a shear component is exerted on the yarn, acting in the direction of the rotor groove. The effect of the shear component is that the yarn twist between the yarn draw-off nozzle and the rotor groove is hindered from leaving that region. With spiral draw-off nozzles constructed in that way, in contrast to draw-off nozzles with a smooth or notched surface, greater twist is imparted to the sliver in the rotor groove, and therefore the number of breaks that occur in the yarn is lowered. Such spiral draw-off nozzles also offer the opportunity of reversing the yarn twist by increasing the yarn draw-off speed, without changing the original yarn breakage numbers, which leads to an increase in production speed.
However, a disadvantage of those known spiral yarn draw-off nozzles is that because of the ribs there is relatively great stress on the yarn, which is not acceptable for all types of yarn.